The present invention relates to an analog mixer circuit for down-converting an RF (Radio Frequency) signal into an IF (Intermediate Frequency) signal or up-converting an IF signal into an RF signal, by mixing an LO (LOcal frequency) signal.
FIG. 4 is a circuit diagram illustrating a conventional mixer circuit. A mixer IC 73 comprises a transistor 50, whereof a collector is coupled to an output terminal 55, a base is coupled to an input terminal 54 and an emitter is coupled to a ground terminal 57. The collector is connected to a power supply terminal 56 through a resistor 51. The base of the transistor 50 is connected to the ground terminal 57 through a resistor 53 and connected to the collector through a resistor 52.
Between the power supply terminal 56 and the ground terminal 57, a power supply voltage Vcc is supplied. The input terminal 54 is connected to an end of a capacitor 61. To the other end of the capacitor 61, an LO signal, which is input to an LO terminal 68, and an IF signal, which is input to an IF terminal 69, are connected through a BEF (Band Elimination Filter) 71 and a BEF 72, respectively. The output terminal 55 is connected to an RF terminal 70 through a capacitor 62 and a HPF (High Pass Filter) 67.
The BEF 71 comprises two capacitors 58 and 59 serially connected between the LO terminal 68 and the capacitor 61, and an inductance 63 grounding a connecting point of the two capacitors 58 and 59. The BEF 72 comprises two inductances 64 and 65 serially connected between the IF terminal 69 and the capacitor 61, and a capacitor 60 grounding a connecting point of the two inductances 64 and 65.
The BEFs 71 and 72 are provided for isolating the LO signal and the IF signal to be supplied to the capacitor 61 from each other, and the circuit constants thereof are so determined that the BEF 71 shows sufficiently high impedance to the IF signal and the BEF 72 shows sufficiently high impedance to the LO signal. Therefore, the LO signal and the IF signal are added to the input terminal 54 of the mixer IC 73 without interfering with each other. The added signal is amplified nonlinearly by the transistor 50 and output to the output terminal 55, wherefrom a high-frequency component is extracted by the HPF 67 to be output through the RF terminal 70.
However, for isolating the LO and the IF signal from each other, the conventional mixer circuit of FIG. 1 necessitates the BEFs 71 and 72, which are inconvenient to be configured on an IC (Integrated Circuit) chip, taking large circuit spaces.
For dealing with this problem, there is disclosed a prior art in a Japanese patent application laid open as a Provisional Publication No. 154019/'96.
FIG. 5 is a circuit diagram illustrating a mixer circuit of this prior art.
In the mixer circuit of FIG. 5, a mixer IC 98 comprises transistors 74, 75 and 76, bias circuits 80 and 81, resistors 77 and 78 and a capacitor 84.
The base of the transistor 74 connected to an LO input terminal 87 of the mixer IC 98 is biased by the bias circuit 80 connected between a power supply terminal 89 and a ground terminal 90, and the base of the transistor 75 connected to an IF input terminal 88 of the mixer IC 98 is biased by the bias circuit 81 also connected between the power supply terminal 89 and the ground terminal 90.
The emitters of the transistors 74 and 75 are connected to the ground terminal 90. The collectors of the transistors 74 and 75 are coupled and connected to the power supply terminal 89 through the resistor 77.
The base of the transistor 76 is connected to the collectors of the transistors 74 and 75. The resistor 78 and the capacitor 84 are connected in parallel between the emitter of the transistor 76 and the ground terminal 90. The collector of the transistor 76 is connected to an output terminal 91 of the mixer IC 98.
An LO signal supplied to an LO terminal 95 is input to the LO input terminal 87 through a capacitor 82, and an IF signal supplied to an IF terminal 96 is input to the IF input terminal 88 through a capacitor 83. A power supply voltage Vcc is supplied between the power supply terminal 89 and the ground terminal 90. Tile output terminal 91 is connected to an end of a HPF 94. The other end of the HPF 94 is connected to an RF terminal 97 through a capacitor 86 and to the power supply terminal 89 through a resistor 79.
In the mixer circuit of FIG. 5, the LO signal input to the LO input terminal 87 of the mixer IC 98 and the IF signal input to the IF input terminal 88 are isolated from each other by the transistors 74 and 75, and mixed signal thereof obtained at coupled collectors of the transistors 74 and 75 is amplified nonlinearly by the transistor 76. The RF signal is extracted by the HPF 94 to be output through the RF terminal 97.
Thus, a mixer circuit is realized in the prior art of FIG. 5 without needing any BEF for isolating input LO and IF signals from each other.
However, the mixer circuit of FIG. 5 necessitates two input transistors 74 and 75 driven in parallel. Hence, power consumption of the mixer circuit of FIG. 5 becomes considerably high.